1. Field
The present invention generally relates to wireless communications systems in which a base station communicates with a number of mobile stations that have position location capabilities, and more particularly to the determination of the position of a base station in a mobile communication network.
2. Description of Related Art
In wireless communication networks, one or more base stations communicate wirelessly with a number of mobile stations (e.g., wireless devices such as cell phones). Mobile stations usually provide standard voice and/or data communication; as an additional feature, some mobile stations have positioning capabilities, which allow the user of the mobile station to determine his or her position. Such position determination capabilities are becoming increasingly useful and important; for example, the regulatory requirements of a jurisdiction may require a network operator to report the location of a mobile station when the mobile station places a call to an emergency service, such as a 911 call in the United States. Or a user may simply want to know his or her position for purposes of locating a nearby restaurant or movie theater.
One well-known type of position location system utilizes satellites in earth orbit to trilaterate a position of the mobile station. One example of such a system is the Global Positioning Satellite (GPS) system currently in operation. Another type of position location system utilizes radio signals from base stations whose locations are known. For example, in one communication network—a Code Division Multiple Access (CDMA) digital cellular network—the position location capability can be provided by Advanced Forward Link Trilateration (AFLT), a technique that computes the location of the mobile station (MS) from the mobile station's measured time of arrival of radio signals from the cellular base stations. An AFLT-enabled wireless mobile station makes pilot phase measurements (PPMs) of the radio signals from the base stations with which it is communicating, and uses these measurements to determine the mobile station's position. A more advanced technique is hybrid position location, where the mobile station also employs a GPS receiver and the position is computed based on both AFLT and GPS measurements.
Message protocols and formats for CDMA position location employing AFLT, GPS, and hybrid receivers, applicable to both the MS-based and MS-assisted cases, have been published in TIA/EIA standard IS-801-1 2001, Position Determination Service Standard for Dual-Mode Spread Spectrum Systems-Addendum. Pages 4-41 through 4-44 of this standard specify that each base station shall transmit a base station almanac message among other elements containing the locations of the base station antennae transmitting the CDMA pilot pseudorandom (PN) sequence.
Thus, in wireless communication systems with AFLT capabilities, the wireless base stations can be used as reference points to assist in fixing the location of a mobile station. However, one prerequisite to using a base station as a reference is precise knowledge of the position of the base station antennae. Timing information regarding the base station is also important. Once known, the base station's antenna position and timing information may be recorded in a base station almanac (BSA) database for use by a position determining entity (PDE). However, acquiring the base station's antenna precise location and timing information can be tedious and expensive.
To further elaborate on the position determination system, data concerning calibration or re-calibration of a base station time offset, base station antenna location and other parameters are typically stored in what is called a “base station almanac.” The base station almanac database provides information for determining an initial position estimate of the mobile station to seed GPS pseudorange searching. Due to PN reuse, the base station almanac database provides information for resolving ambiguities about which observed pseudorandom noise sequences (PNs) correspond to which physical sectors of a CDMA network base station. The base station almanac database provides the cellular base station sector antenna position from which signals emerge. AFLT range measurements are made to these antenna positions.
In some instances the position of a base station antenna may change slightly or by a larger distance, and in that instance the corresponding almanac information must be updated. For example a base station antenna may be relocated, or a base station transceiver may be repaired or replaced, a new cable may be placed between a transceiver and an antenna causing a change in the base station antenna position or timing information. In another example, a base station can be logically (but not physically) moved, when, for example, two physical base stations swap their identification information, and in such a circumstance, the two stations would appear to have swapped locations although neither base station physically moves.
In such situations, if not updated, the corresponding information in the database can become erroneous; therefore the almanac database must be updated before resuming service from the base station. However, updating the almanac database using conventional techniques can be time-consuming and expensive: for example conventionally the antenna location may be determined by a survey or with reference to coordinates read from a map, and if the antenna coordinates are entered manually into the database then the possibility of human error arises. The base station timing information is also subject to human error in cases where custom hardware is used to measure the timing information, and timing offsets are entered manually into the database.
In Local Area Networks the significant number of base stations (several million), the deployment practices and the ease of moving the portable base station from one physical location to another make it impossible to locate and maintain the base station almanac using conventional surveying techniques.
In summary, deployment of base station time difference of arrival ranging solutions such as AFLT or hybrid-AFLT requires accurate base station almanac (BSA) information including base station antennae locations and forward link calibration parameters such as time corrections, which are key components of the BSA information. Today, population of the BSA parameters is done individually for each base station, which is a manual, laborious and expensive process, and therefore hinders the deployment and proliferation of LBS services. It would be an advantage to provide a practical system that can update the BSA parameters in real time using data from mobile devices that are in the area to compute base station locations and forward link calibration parameters.